Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, NY) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single waveguide, each channel being assigned a particular wavelength. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical networks.
To provide compatibility of WDM systems with existing networks, it will be desirable to convert a signal from a received transmission wavelength from a customer to a specific channel wavelength within the WDM system. This is particularly true in WDM systems employing many channels, often referred to as "dense" WDM, where channel spacings are on the order of one nanometer or less. Such WDM systems require precise control of the optical signal wavelength for each channel in order to avoid "crosstalk," i.e., interference between adjacent channels.
Previously, attention has been focused on conversion of a single transmission channel from a wavelength outside the wavelength band amplified by optical amplifiers to a wavelength within the wavelength band amplified by optical amplifiers. U.S. Pat. No. 5,267,073 describes wavelength conversion in a conventional single channel optical system to enable signal amplification by optical amplifiers. In the patent, an adapter is provided to receive a transmission optical signal having a wavelength which is outside the operating parameters of the optical amplifier. The signal is supplied to an optical-to-electronic converter module. The resultant electrical signal is output to an adjusting means comprising a laser piloting circuit for directly modulating a signal laser. The output of the signal laser is subsequently amplified by an optical amplifier.
Direct modulation of lasers, particularly semiconductor diode lasers, as described in the '073 patent, is disadvantageous for high data-rate, multiple channel WDM optical communication systems. In direct modulation, the spectral line width of the laser cavity is broadened as a result of frequency chirp. Consequently, the spectral line width of the optical carrier is broadened. Because optical carrier signal spacing is critical in a WDM system, excessive spectral line broadening is unacceptable. Additionally, for long-haul systems, frequency chirp and fiber chromatic dispersion combine to produce pulse spreading, obscuring the information carried by the optical signal.
Thus, there is a need in the art for improved WDM optical communication systems which can receive incoming optical transmission signals and place the information from the transmission signals onto optical channels within the WDM system.